Resorption of bone is the unique function of the osteoclast, a cell type derived from hematopoietic precursor cells in the myeloid lineage. Osteoclast development requires the activation of cell surface receptors on preosteoclasts, with an absolute requirement for stimulation of RANK (receptor for activation of NFkappa b) and c-fms (receptor for macrophage colony stimulating factor, M-CSF). Other immunomodulatory receptors that influence osteoclast development and function are less well understood. Others and we have recently demonstrated expression and function of a novel family of receptors in osteoclasts, the DAP12-associated receptors. In our preliminary work, we demonstrate the presence of DAP12 and DAP12 associated receptors in preosteoclasts and osteoclasts and show that osteoclasts developed in vitro from DAP12 -/- mice are defective in osteoclast development with decreased bone resorption. In vivo, the tibias of DAP12 -/-mice show increased bone mass. DAP12-associated receptors are defined by their functional association with the signaling adapter protein DAP12 (DNAX adapter protein 12) and mediate cellular activation and maturation in other myeloid lineage cells (macrophages and dendritic cells). We suggest a similar role for P12 in osteoclasts. Interestingly, a rare inherited human disease Polycystic Lipomembranous Osteodysplasia with Sclerosing Leukoencephalopathy (PLOSL) involving bony abnormalities is associated with loss of function of the DAP12 gene. PLOSL patients have multiple bony cysts, pathological fractures and severe arthritis before age 30. Recent studies have shown that cells from PLOSL patients show abnormal in vitro osteoclast development similar to our observations in DAP12 -/-mice. Our studies will further examine the hypothesis that signals mediated by DAP12 and DAP12-associated receptors (DAR) regulate the development, activation and survival of osteoclasts and play a role in bony remodeling. Specific Aims: 1) Examine the effect of DAP12 and DAR activation on osteoclast (OC) multinucleation 2) Examine the effect of DAP12 and DAR activation on osteoclast function 3) Examine the effect of DAP12 and DAR activation on osteoclast (OC) survival 4) Further examine the bony phenotype of DAP12-/- animals by bone histomorphometry and microCT analysis and examine the response to distinct bone resorptive stimuli 5) Examine the effect of DAP12 signals on RANK signaling intermediates and cytoskeletal rearrangement.